The present invention relates to a method of manufacturing a surface shape recognition sensor and, more particularly, to a method of manufacturing a surface shape recognition sensor that senses a fine three-dimensional pattern such as a human fingerprint or an animal muzzle pattern.
Concern for the security technique has been increasing in the recent social environment along with the development of the information-oriented society. For example, in the information-oriented society, a holder verification technique for constructing a system such as an electronic cashing system has become an important key. Also, studies and developments have become active on the verification technique which prevents a robbery or illegal use of a card (for example, Yoshimasa Shimizu et al., “A Study on the Structure of a Smart Card with the Function to Verify the Holder”, Technical Report of IEICE, OFS92-32, pp. 25–30 (1992).
The verification scheme for preventing illegal use includes various types that utilize a fingerprint or voiceprint. Among them, many technical developments have been made on the fingerprint verification technique. The scheme of detecting the shape of the fingerprint when verifying the fingerprint is roughly divided into an optical reading scheme and a scheme of detecting the three-dimensional pattern of the skin surface of the human fingertip by converting it into electrical signals.
According to the optical reading scheme, the fingerprint is input as optical image data and is collated by mainly using light reflection and a CCD image sensor (Japanese Patent Laid-Open No. 61-221883). As another scheme, one that utilizes a piezoelectric thin film in order to read the pressure difference of the fingerprint has also been developed (Japanese Patent Laid-Open No. 5-61965). Similarly, as a scheme that detects the shape of the fingerprint by converting a change in electric characteristics caused by contact with the skin surface into the distribution of electrical signals, a scheme that detects a resistance change amount or capacitance change amount using a pressure-sensitive sheet has been proposed (Japanese Patent Laid-Open No. 7-168930).
Of the techniques described above, however, the scheme that uses light is difficult in downsizing and versatile applications, so that its application is limited. The method that senses the three-dimensional pattern of the finger by using a pressure-sensitive sheet or the like is difficult in practical use because the material is specific and the sheet is difficult to process. Hence, this scheme may lack reliability.
A capacitive fingerprint sensor has been developed which is fabricated by using the LSI manufacturing technique (Marco Tartagni and Roberto Guerrieri, “A 390 dpi Live Fingerprint Imager Based on Feedback Capacitive Sensing Scheme”, 1997 IEEE International Solid-State Circuits Conference, pp. 200–201 (1997)). According to this sensor, the three-dimensional pattern of the skin surface is detected with small sensors arrayed two-dimensionally on an LSI chip by utilizing the feedback electrostatic capacitive scheme.
In the capacitive sensor, two plates are formed on the uppermost layer of the LSI interconnection, and a passivation film is formed on the plates. When a fingertip comes into contact with this sensor, the skin surface serves as the third plate, and the three-dimensional pattern on the skin surface is isolated by an insulating layer comprised of air. Hence, the fingerprint is detected by sensing the capacitances that differ from one sensor to another depending on the difference in distance from the sensor surface. As compared to a conventional optical-scheme sensor, this structure needs no specific interface and can be downsized as its characteristic features.
The fingerprint sensor is in principle obtained by forming a plurality of sensor electrodes on a semiconductor substrate to form a matrix, and forming a passivation film on the sensor electrodes. The capacitance between the skin surface and the sensor is detected through the passivation film, thus detecting the fine three-dimensional pattern.
This conventional capacitive fingerprint sensor will be briefly described with reference to the accompanying drawings. As shown in FIG. 4A, this fingerprint sensor has interconnections 403 on a semiconductor substrate 401 with LSIs or the like through a lower insulating film 402, and an interlayer dielectric film 404 on the interconnections 403.
Sensor electrodes 406 having, e.g., a rectangular planar shape, are formed on the interlayer dielectric film 404. The sensor electrodes 406 are connected to the interconnections 403 through plugs 405 in through holes formed in the interlayer dielectric film 404. A passivation film 407 is formed on the interlayer dielectric film 404 to cover the sensor electrodes 406, thus forming sensor elements. The plurality of sensor elements with the above arrangement are two-dimensionally arranged such that the sensor electrodes 406 of the adjacent sensor elements do not in contact with each other, as shown in FIG. 4B.
The operation of this capacitive sensor will be described. In fingerprint detection, a finger as the fingerprint detection target comes into contact with the passivation film 407. When the finger comes in contact with the surface of the passivation film 407, on the sensor electrodes 406, the skin surface that has come into contact with the passivation film 407 serves as an electrode, and forms a capacitance with the sensor electrodes 406. This capacitance is detected by a detector (not shown) through the interconnections 403. As the fingerprint on the fingertip is formed of the three-dimensional pattern of the skin surface, when the finger comes into contact with the passivation film 407, the distance between the skin surface as the electrode and the sensor electrodes 406 differs between projections and recesses that form the fingerprint.
This difference in distance is detected as a difference in capacitance. Therefore, when the distribution of the different capacitances is detected, it corresponds to the shape of the projections of the fingerprint. Namely, this capacitive sensor can sense the fine three-dimensional pattern of the skin surface.
As compared to a conventional optical-scheme sensor, the capacitive fingerprint sensor requires no specific interface and can be downsized.
The capacitive sensor described above can be simultaneously mounted on, e.g., the following integrated circuit (LSI) chip. For example, the capacitive sensor described above can be simultaneously mounted on an integrated circuit chip where a storage which stores fingerprint data for collation and a recognition processor which compares and collates the fingerprint data prepared in the storage and a read fingerprint are integrated. In this manner, when the capacitive sensor is formed on one integrated circuit chip, information temper or the like in data transfer among different units becomes difficult, and the performance of information secrecy can be improved.
In the sensor described above, however, as the skin surface is utilized as the electrode, the LSI which is mounted simultaneously is likely to cause electrostatic breakdown due to static electricity produced during contact. Hence, conventionally, a sensor, for which the stability, sensitivity, reliability, and the like are considered and furthermore downsizing and versatility are considered and which senses a fine three-dimensional pattern such as a human fingerprint or animal muzzle pattern, and a method of manufacturing the same have been sought for.